The storage of hydrogen in metal hydrides is based on chemisorption, i.e. no molecular hydrogen (H2) is stored but the hydrogen reacts with the metal to form metal hydrides. Storage of hydrogen in the form of metal hydrides has the advantage over storage of hydrogen in, for instance, liquid or compressed state in that it does not require the use of low temperatures or high pressures.
The hydrogen compounds of the highly electropositive s-block elements are non-volatile, electrically non-conducting, crystalline solids generally containing hydride (H) anions. These properties, as well as their structures, lead to their classification as saline (salt-like) hydrides. The binary Group 1 metal hydrides share the crystal structure of sodium chloride (a face-centred cubic array of anions in which the cations occupy the octahedral holes). Lithium hydride (LiH) contains 12.5 wt % hydrogen, but requires 910° C. for an equilibrium pressure of 105 Pa (1 bar). Because of its stability, LiH has not been considered as a practical hydrogen storage material.
Magnesium hydride (MgH2) shares the crystal structure of rutile (a distorted hexagonal close-packed anion lattice in which the cations occupy half the octahedral holes). Magnesium hydride is one of the most studied materials for hydrogen storage mainly due to its high hydrogen content (7.6 wt %) and low manufacturing cost. However, its slow hydrogen absorption/desorption kinetics and high dissociation temperature of nearly 300° C. at 105 Pa (1 bar) H2 pressure limit its practical applications.
In 1978, Ashby et al. (Inorg. Chem., 1978, 17, 322-326) prepared and characterized by X-ray diffraction the compounds LiMgH3, LiMg2H5 and Li2MgH4 by mixing organolithium or mixed organolithium/magnesium compounds with LiAlH4.
Relatively few theoretical studies exist on LiMgH3: Li et al. (Phys. Rev. B, 1991, 44, 6030-6036) calculated the lattice spacing and the electronic structure and reported that LiMgH3 is an insulator. Khowash et al. (Phys. Rev. B, 1997, 55, 1454-1458) calculated the cohesive energy and the electron density. Vajeeston et al. (JALCOM, 2008, 450, 327-337) predicted that the most stable arrangement for LiMgH3 is the trigonal LiTaO3-type structure. They also considered the formation energy of this compound and suggested a route to synthesis or stabilize LiMgH3. The proposed pathway is from elemental lithium and magnesium in a hydrogen atmosphere.
Goto et al. (JALCOM, 404-406, 2005, 448-452) prepared LiMg2H5 under high pressure, typically 2-5 GPa at 973 K by using an anvil-type apparatus. This hydride exhibits a primitive cubic-type structure.
U.S. Pat. No. 2,961,359 published on 22 Nov. 1960 discloses magnesium alloy materials which retain strength at high temperatures for use in aircraft construction and the like. These alloys are based on crystalline hydrided lithium-magnesium alloys and are made by heating a lithium-magnesium alloy in a hydrogen atmosphere until substantially all the lithium is converted to a stable strengthening precipitate of finely divided lithium hydride dispersed in an essentially magnesium matrix. This annealing process promotes a crystalline structure which provides a hardening effect for the magnesium alloy, so that it is stable up to high temperature, since the lithium hydride is stable to 680° C. U.S. Pat. No. 2,961,359 thus provides an alloy of magnesium that retains its strength at elevated temperatures by exploiting the stability of the lithium hydride. Because the composition of U.S. Pat. No. 2,961,359 is crystalline (which is what gives it its strength), the composition is not suitable for reversibly storing hydrogen, since the lithium hydride is stable.
JP-A-2003-073765 discloses a hydrogen storage material with a body-centred cubic lattice crystal structure prepared by alloying magnesium, which has a close-packed hexagonal crystal lattice, with lithium, having a body-centred cubic lattice. As long as the resulting alloy has a body-centred cubic lattice, there is then a tetrahedral site available for occupancy by a hydrogen atom. However, to enable hydrogen absorption, it is then necessary to incorporate hydrogen dissociating substances, at least in the surface of the magnesium-lithium system, such as nickel, nickel alloys, palladium, palladium alloys, rare-earth hydrogen storage alloys, and titanium hydrogen storage alloys. These additional metallic components dissociate molecular hydrogen into atomic hydrogen to enable its absorption, and only small absorption of hydrogen is observed if insufficient hydrogen dissociating materials are present.
JP-A-2008-190004 discloses a wide range of magnesium-based hydrogen storage alloys of general formula Mg1-xMxHy wherein M may be at least one chosen from the group consisting of Li, Na, K, Rb, Ca, Sr, Ba, Sc, Ti, Zr, Hf, V, Nb, Ta, and Pd, where x=0.04 to 0.8 and y=0.2 to 2. Although various examples are given, a simple magnesium-lithium system is not described. Furthermore, the desirable goal of reduction in temperature for hydrogen desorption and/or in occlusion starting temperature is met by incorporating additional elements or compounds. The samples for which X-ray diffraction data are given show that the hydrogen storage materials are essentially crystalline.
There is still a need in the art for a hydrogen storage material that allows a reversible storage of hydrogen at low hydrogen uptake, low release temperatures and under mild rehydriding conditions.